As alloys which have high strength and high corrosion resistance and which are used at temperatures around 1000° C., there have been known alloys into which Cr (chromium) for improving corrosion resistance and W (tungsten) for improving strength are added. When these alloys, e.g., commercial Ni-33Cr-15W alloys, are exposed at a high temperature to an environment including Cl (chlorine) and S (sulfur) having high concentrations, they are subjected to extremely large corrosion damage. Particularly, since the environment includes Cl, corrosion is considerably promoted. Because chlorides have a low melting point and a high volatility, a protective scale cannot be formed against corrosion, so that chlorides cause considerably large corrosion damage to metallic materials. Conventionally, metallic materials cannot be used as they are in a high temperature, strongly corrosive environment including chlorides, and therefore metallic materials may be cooled to prolong their lifetime.
In view of reducing corrosion, it is desirable to cool a metallic material to lower the temperature of metal in order to prolong lifetime. However, for the purpose of using in an environment around 1000° C., cooling is not desirable because the temperature of the atmosphere is lowered.
A corrosion test was taken for the purpose of measuring corrosion resistance in an environment including HCl (hydrogen chloride). A commercial Ni-33Cr-15W alloy (also referred to as “T alloy”) which has been used as an alloy having high strength at high temperatures and high corrosion resistance was used as a specimen. A gas including HCl (N2-10% O2-1000 ppm) was flowed over the specimen, and in this state, the specimen was held at 900° C. for 200 hours. FIG. 6 is microphotographs of cross-sections of the T alloy specimen after the test. The T alloy had strong internal corrosion, and when the specimen was polished to a mirror finish and left in a room, sweat was formed on a surface of the specimen. This is because chloride included in the alloy has deliquescence such that it is coupled to moisture in air to form sweat on the surface of the specimen. Thus, chlorination occurs, and materials are subject to considerable corrosion.